The present invention relates to an improvement in the temperature characteristics of gear pumps or motors.
Generally in the operation of a gear pump or motor, if there are more clearances than are needed between the tooth crests of the two interlocking gears and the inner wall surfaces of the gear bower, the liquid on the high pressure side will leak in a large amount through these clearances to the low pressure side. This will not only lower the volumetric efficiency of the gear pump or motor significantly but also generate heat which could result in seizure.
It is common practice, therefore, that in order to keep the tooth crests of the two gears in contact with, at least, some part of the inner wall surfaces of the gear bower during operation, the addendum circle diameter of the gears, inner diameter of the gear bower, the bearings supporting the respective gear shafts and other assembly parts are optimized beforehand and, after assembly of the gear pump or motor, the break-in operation is performed. In the break-in operation, the two gears are pressed by the liquid on the high pressure side to the low pressure side so that the inner wall surfaces are cut off at the low pressure side by the tooth crests of the two rotating gears. In this way, by applying a break-in to a stipulated position of the gear bower, a minimized radial clearance is secured in the operation of the gear pump or motor and thus, even if there are some manufacturing errors with regard to dimensions of the respective assembly parts, the tooth crests of the two gears are kept in contact with the inner wall surfaces at the low pressure side of the gear bower. This seals the liquid which tends to leak from the high pressure side to the low pressure side.
Under these circumstances, it has been established as common sense, among those who are skilled in the art, that the housing may not be made of a material of the same type as used for the two gears which are made of a ferrous material. The reason is that it was believed impossible to apply a break-in having good smooth surfaces on the grounds that built-up edges are formed on the tooth crests of each gear, made of a ferrous material, since they cut into the inner wall surfaces of the gear bower, which is made of a material of the same ferrous type, during the break-in operation. This results in burrs and scratches on the cut surfaces of the gear bower.
Conventionally, therefore, the housing has been made of aluminum alloy materials which produce no built-up edges on the tooth crests of the two gears when they cut the inner wall surfaces of the gear bower. By using such a housing made of aluminum alloy materials, it is certainly possible to give a good break-in to the inner wall surfaces of the gear bower and consequently to fabricate gear pumps or motors having apparently a good volumetric efficiency and no chance of seizure.
Nevertheless, it has been appreciated that even such gear pumps or motors have still the following defect depending on their working conditions. Namely, if the gear pump or motor is used at a lower temperature after the break-in operation at room temperature, such a temperature change will cause a larger shrinkage in the housing made of an aluminum alloy material than in the gears made of a ferrous material, due to the difference in heat expansion coefficient between these materials. Consequently tooth crests of each gear will cut deeper inner wall surfaces of the gear bower. Therefore, when the gear pump or motor operates again at room temperature or a high temperature, the radial clearance becomes larger than needed due to the difference in heat expansion coefficient between the gears and housing, so that the internal liquid leakage flowing from the high pressure side to the low pressure side may increase, thus not only lowering the volumetric efficiency significantly but also causing a seizure hazard in some cases.
On the other hand, in order to resolve this defect, inherent in aluminum alloy materials, it is essential to make the housing out of a ferrous material, the same as the two gears. However, since built-up edges are formed on tooth crests of each gear when the break-in is applied, it has been impossible up to now to have smoothly cut surfaces, which left the technical problem that no housings made of ferrous materials were usable.